1. Field of the Invention
This invention relates generally to a method of manufacturing drill bits and other drilling-related structures generally used for drilling subterranean formations and, more specifically, to a method of manufacturing a drill bit or drilling-related structure having a porous, sintered steel powder core and a powdered tungsten carbide (WC) shell commonly infiltrated with a hardenable liquid binder. In a preferred embodiment, a sintered, preformed blank is formed and placed in a mold configured as a bit or other drilling-related structure, the preformed blank being sized to provide space between the blank and the mold wall to accommodate a layer of WC powder therebetween.
2. State of the Art
A typical rotary drill bit includes a bit body secured to a steel shank having a threaded pin connection for attaching the bit body to a drill string, and a crown comprising that part of the bit fitted with cutting structures for cutting into an earth formation. Generally, if the bit is a fixed-cutter or so-called "drag" bit, the cutting structures include a series of cutting elements formed at least in part of a super-abrasive material, such as polycrystalline diamond. The bit body is generally formed of steel, or a matrix of hard particulate material such as tungsten carbide (WC) infiltrated with a binder, generally of copper alloy.
In the case of steel body bits, the bit body is typically machined from round stock to the desired shape. Internal watercourses for delivery of drilling fluid to the bit face and topographical features defined at precise locations on the bit face may be machined into the bit body using a computer-controlled, five-axis machine tool. Hard-facing for resisting abrasion during drilling is usually applied to the bit face and to other critical areas of the bit exterior, and cutting elements are secured to the bit face, generally by inserting the proximal ends of studs, on which the cutting elements are mounted, into apertures bored in the bit face. The end of the bit body opposite the face is then threaded, made up and welded to the bit shank.
In the case of a matrix-type bit body, it is conventional to employ a preformed, so-called bit "blank" of steel or other suitable material for internal reinforcement of the bit body matrix. The blank may be merely cylindrically tubular, or may be fairly complex in configuration and include protrusions corresponding to blades, wings or other features on and extending from the bit face. Other preform elements or displacements comprised of cast resin-coated sand, or in some instances tungsten carbide particles in a binder, may be employed to define internal watercourses and passages for delivery of drilling fluid to the bit face, as well as cutting element sockets, ridges, lands, nozzle displacements, junk slots and other external topographic features of the bit. The blank and other displacements are placed at appropriate locations and orientations in the mold used to cast the bit body. The blank is bonded to the matrix upon cooling of the bit body after infiltration of the tungsten carbide with the binder in a furnace, and the other displacements are removed once the matrix has cooled. The upper end of the blank is then threaded and, made up with a matingly threaded shank, and the two welded together. The cutting elements (typically diamond, and most often a synthetic polycrystalline diamond compact or PDC) may be bonded to the bit face during furnacing of the bit body if thermally stable PDC's, commonly termed TSP's (Thermally Stable Products), are employed, or may be subsequently bonded thereto, usually by brazing or mechanical affixation.
As may be readily appreciated from the foregoing description, the process of fabricating a matrix-type drill bit is a somewhat costly, complex, multi-step process requiring separate fabrication of an intermediate product (the blank) before the end product (the bit) can be cast. Moreover, the blanks and preforms employed must be individually designed and fabricated.
The mold used to cast a matrix body is typically machined from a cylindrical graphite element. For many years, bit molds were machined to a general bit profile, and the individual bit face topography defined in reverse in the mold by skilled technicians employing the aforementioned preforms and wielding dental-type drills and other fine sculpting tools. In more recent years, many details may be machined in a mold using a computer controlled, five-axis machine tool. In some cases, the mold fabrication process has been made faster and less costly by use of rubber displacements duplicating in fine detail the topography of an entire bit profile and face, which displacements are then used to cast a ceramic bit mold of appropriate interior configuration, which is then used to cast a bit.
While matrix-type bits may offer significant advantages over prior art steel body bits in terms of abrasion and erosion resistance, and while recent advances in matrix technology have markedly increased the toughness and ductility of matrix bodies, in many cases the higher cost of a matrix-type bit and the longer time to fabricate same may result in the customer choosing a cheaper steel body bit with a faster delivery time. In either case, the customer must choose between a tough but less abrasion-resistant bit and a more expensive, highly abrasion-resistant bit with reduced toughness.
Both batch and conveyor-type continuous furnaces, induction heating coils, and other heating methods known in the art may be used to supply the heat necessary for sintering to occur. It is well recognized in the art to use sintering techniques to sinter and forge mixtures of steel powder and tungsten carbide to form inserts for rock-cutting bits, such as the method disclosed in U.S. Pat. No. 4,484,644 to Cook et al. It has also been recognized in the art to replace at least a portion of the hard metal matrix material (WC) of a typical bit with a tougher, more ductile displacement material, such as iron, steel, or alloys thereof. As described in U.S. Pat. No. 5,090,491 to Tibbitts et al., it is desirable to substitute a less expensive displacement material (such as steel at about 50 cents per pound) for the more expensive hard metals like tungsten carbide (at about ten dollars per pound) to provide a finished bit with improved toughness and ductility as well as impact strength. However, this reference provides that the displacement material should preferably be a mesh size of at least 400 (approximately 0.001 inches) and also states that very fine powdered materials (i.e., less than 0.001 inches in diameter) such as iron may sinter and shrink during fabrication, it being undesirable for the powder to shrink substantially during the heating process. Likewise, in GB 1,572,543 to Holden, the use of relatively inexpensive materials to provide the metal matrix of a bit, such as iron powder bonded with a copper based alloy, is disclosed. Nowhere, however, do any of these references suggest that a powdered steel blank be sintered or otherwise preformed, then subsequently infiltrated along with a layer of tungsten carbide powder to form a bit or drilling-related structure.
It is known in the art that although hard, the strength, and more particularly the ductility, of cemented hard-carbide articles are almost always inferior to those obtained by casting or forging steel, although better properties can be obtained from carbides by special means, such as additional working and annealing. Thus, it would be advantageous to provide a method of manufacturing a bit or other drilling-related structure that is a relatively simple process and that reduces the cost of producing the structure by replacing a significant amount of the bit matrix material of a typical drilling structure with a sintered steel powder blank without sacrificing the bit's resistance to erosion and abrasion. Moreover, it would be advantageous to provide such a drilling structure that has improved toughness and impact strength over similar structures manufactured by prior art methods.